1. Field of the Invention
The present invention relates to a red phosphor, and in particular relates to a white light illumination device utilizing the same.
2. Description of the Related Art
The white light illumination device has been gradually replaced from conventional tungsten lamps or fluorescent lamps to commercially available light emitting diode (herein after LED). The LED has advantages described as follows: (1) its small size is suitable in illumination of array packaging and collocated with different colors if necessary; (2) a relatively long life of more than 10,000 hours and 50 times that of the conventional tungsten lamp; (3) durability due to transparent resin applied as packaging resin, thereby enhancing shock resistance; (4) its interior structure is free of mercury, such that the LED is environmental friendly and does not have problems such as pollution and waste management; (5) saves energy and consumes low electric power, wherein the electric power consumption of the LED is ⅓ to ⅕ that of the conventional tungsten lamp.
Generally, white light is a mixture of at least one colored light. For example, the white light seen by a human eye can be formed by mixing blue and yellow lights or mixing blue, green, and red lights. The former white light is a two-wavelength, and the latter is three-wavelength white light.
The three most common commercially available semiconductor white light devices are described as follows. The first is a white illumination device collocated by red, green, and blue LED chips. This white light module has high luminescence efficiency and high color rendering. However, the different color LED chips require different epitaxial materials, wherein different electrical voltages are needed. Accordingly, the manufacturing cost is high, the circuit layout is complicated, and the appropriate mixing of different color lights is difficult.
The second is a white illumination device disclosed by Nichia Corporation. The most common version is the white light formed by a yellow YAG phosphor excited by a blue LED. The periphery of the blue LED is filled with optical gel sealing the yellow YAG phosphor. The blue LED emits a blue light having a wavelength of about 400 nm to 530 nm. The yellow YAG phosphor is excited by a part of the blue light and then emits a yellow light. The remaining part of the blue light collocates with the yellow light to form the two-wavelength white light.
The described two-wavelength (blue and yellow) white LED has many illumination limitations. Because in this kind of two-wavelength white light, the color temperature is usually high and the color is not uniform. Therefore, the collocation of the blue light and the yellow phosphor needs to be improved to have higher color quality. Next, because the blue light wavelength from LED chip will change along with different temperatures, the color control of the white light is difficult. In addition, the two-wavelength white light lacks red light, thereby reducing the color rendering.
The third white illumination device is formed by blue, green, and red phosphors evenly dispersed in optical gel. By excitation, the phosphors emit red, green, and blue lights which further collocate to provide a three-wavelength white light. Although the luminescence efficiency thereof is relatively lower, the three-wavelength white light has higher color rendering. The manufacturing flexibility and properties of the third white illumination device is comparably advantageous over the first and second commonly found white illumination devices. Most phosphors are sulfides, nitrides, or oxides. Because sulfide phosphors usually have high luminescence efficiency but low stability, they are easily degraded by moisture or oxygen. On the other hand, the nitride phosphors have high stability, but the synthesis of the nitrides is difficult such as in high temperature/pressure condition, thereby increasing costs and risks. The most general phosphors are oxides such as silicates or aluminates etc., but suitable silicate red phosphors are scarce at present. Therefore, an invention for a good silicate red phosphor is needed and further applied in white illumination devices.